Currency validators are designed to receive a banknote and move the banknote through an evaluator channel prior to accepting and storing of the accepted banknote. From time to time a banknote can become jammed in the validator which creates problems, particularly for unsupervised installations.
Jamming of a validator often is due to a wet or high humidity banknote or due to high density paper sometimes found in fraudulent banknotes. These conditions can be recognized by a capacitor sensor.
When the currency bill passes between capacitor electrodes, the capacitor capacitance increases according to the effect of the dielectric properties of the currency note. The deviations from this value will be observed when the samples with higher or lower density are tested in the validator.
Water has a dielectric constant almost ten times higher than the dielectric constant of currency paper. When we, currency paper passes between the capacitor plates, its capacitance is higher than dry paper, the wetter the paper, the larger the capacitance (as compared to the authentic currency paper). Therefore, the capacitive sensor can determine the "humidity" of currency paper and can be used to evaluate the authenticity of the paper, as the currency paper is being evaluated by the validator.
Many validators are used in a generally non-supervised application such as a vending machine. Fraudulent bills often have a high density and if fully processed by a validator, can become jammed or damage the validator.
It is important in validators to reject fraudulent bills, however, it is also important to reject bills which may become jammed in the validator or which may damage the validator. A jammed validator causes the operator problems and also frustrates the user.
Information about the humidity and other parameters of the paper, evaluated by a validator, are important for the validator's operation.
The design of automatic validators makes contradictory demands. The size of the sensor should be small. It should be designed in such a way that it can be placed anywhere inside the validator channel. Rigid mechanical and electrical connections between the sensor elements placed on the opposite sides of the validator channel lead to complex configurations. The measurement results should not significantly vary with wobble of the paper in the validator channel. It is also desirable for the validator to reject bills which are likely to become jammed in the validator.